1. Field of the Invention
This invention relates to a method and device for code modulation, method and device for code demodulation, and method and device for code decoding, and particularly relates to a method and device for code modulation, method and device for code demodulation, and method and device for code decoding suitably used for recording and reproducing audio data and video data in a recording media such as a disk or tape in a form of digital data.
2. Description of Related Art
High density recording of video and audio digital data in a recording media such as a magnetic disk, optical disc, magneto-optic disc, and magnetic tape is desirable. Usually, high density recording is realized using partial response (1, 0, -1) or partial response (1, 1). Higher density recording is realized using PRML (Partial Response Maximum Likelihood) that is a combination of partial response (1, 0, -1) or partial response (1, 1) with Viterbi decoding.
For example, in partial response (1, 0, -1) (abbreviated as PR4 hereinafter), assuming that the free squared Euclid distance (referred to as geometrical distance hereinafter) in the conventional peak (bit by bit) detection method is 1, then the geometrical distance for decoding can be 2 by performing Viterbi decoding. The larger geometrical distance can improve S/N ratio, and provide a means for additional high density recording. Further, PR4 allows the geometrical distance for Viterbi decoding to be 4 depending on the code modulation method.
For reproduction, for example, in order to suppress the generation of error due to the fluctuation of a reference level when a reproduction signal is converted to a binary code (digitized), or in order to suppress the generation of fluctuation in various error signals such as a tracking error signal in so-called servo control of a disk device or tape device, it is required to not include a direct current component in a modulation code.
Symbols of data 1! and 0! are represented by +1 and -1, then so-called DSV (Digital Sum Value), that is the grand total of symbols from the starting point of a modulation code train, is an index for evaluation of the above-mentioned direct current. A smaller absolute DSV represents a less direct current component or a low frequency component.
In modulations such as 8-10 modulation used for digital audio tape recorders (DAT), EFM (Eight to Fourteen Modulation) used for compact disc (CD) players, and Miller 2 (Miller Square) used for magnetic disk devices, DSV control is employed for reducing the absolute value of DSV, for example, after NRZI (Non Return Zero Inverted) that is so-called mark length modulation.
In PR4, DSV control may not be performed for reducing the absolute value of DSV of a modulation code train because PR4 has no direct current component in nature. But in partial response (1, 1) (abbreviated as PR1 hereinafter), DSV control should be performed to a modulation code train because PR1 a has direct current component by nature.
Further, application of a PRML system involves a problem of the length of path memory when Viterbi decoding is used. The path memory is a memory device for storing an estimated value of decode until the result of Viterbi decoding is established, and has a length proportional to the time length until the result is established. The time length until the establishment of the result, or the length of path memory, can be controlled dependent on the method for modulation coding. For example, the maximum value of the successive number of 1's in a modulation code train before NRZI modulation is the determinant of the required length of the path memory for application of 8-10 modulation code to PR1.
For a real device, it is not possible to provide an infinite path memory, and it is required to provide a path memory as short as possible in order to save cost and occupied space. A control for shortening the length of a path memory should be performed to a modulation code train.
In PR1 which has small spectrum in high frequencies and is suitable for high density recording in comparison with PR4, it is possible to simultaneously perform DSV control and the conversion of geometrical distance to 2 when decoding and to shorten the length of path memory by using the improved 8-10 modulated code proposed in the patent specification and drawings of U.S. patent application Ser. No. 08/309981 (Filing Date : Oct. 13, 1992) this applicant. The improved 8-10 modulation can not perform simultaneous DSV control and conversion of geometrical distance to 2 when decoding and can not shorten the length of path memory. This is disadvantageous. Though the geometrical distance when decoding can be 2, the geometrical distance when decoding can not be larger than 2. This is disadvantageous.
In order to solve the problem, the inventors of the present invention have proposed a code modulation method in which 3*3 states are located in the space of a transition diagram composed of the first axis and second axis. A code 00! is assigned for transition from a certain state to another state along the first axis in the first direction. A code 11! is assigned for transition from a certain state to another state along the first axis in the second direction opposite to the first direction. A code 01! is assigned for transition from a certain state to another state along the second axis in the third direction. A code 10! is assigned for transition from a certain state to another sate along the second axis in the fourth direction opposite to the third direction to generates a 12-bit code. The 12-bit code is assigned to 8-bit data correspondingly. Thereby a 8-bit data is converted to a 12-bit code. This proposed code modulation method and code demodulation method are described in the patent specification and drawings of U.S. patent application Ser. No. 08/579,002 (Filing Date: Dec. 27, 1995). (Japanese Patent Application corresponding to this application is not yet published when this application was filed in the U.S.A. Therefore, the application is described herein only for reference to an examiner)
However, this method performs not so high of conversion efficiency because this method is a method for converting a 8-bit data to a 12-bit code. This method is still not sufficient in the aspect of high density recording and reproduction.